1. Field of the Invention (Technical Field)
The present invention relates to catalysts in mesoporous structures.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
The following references discuss silica and/or catalyst chemistry: xe2x80x9cThe Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry,xe2x80x9d R. K. Iler, Wiley, New York, 1979; xe2x80x9cAttrition-Resistant Porous Particles Produced by Spray-Drying,xe2x80x9d H. E. Bergna, Characterization and Catalyst Development: An Interactive Approach, American Chemical Society, 1989, 55-64; xe2x80x9cOrdered Mesoporous Molecular-Sieves Synthesized by a Liquid-Crystal Template Mechanism,xe2x80x9d Kresge, et al., Nature, 1996, vol. 359, 710; xe2x80x9cActivation of Precipitated Iron Fischer-Tropsch Catalysts,xe2x80x9d Shroff, et al., J. Catalysis, 1995, vol. 156, 185; xe2x80x9cMesoporous Silica Synthesized by Solvent Evaporation: Spun Fibers and Spray-Dried Hollow Sphere,xe2x80x9d Bruinsma, et al., Chem. Mater., 1997, vol. 9, 2507; xe2x80x9cAerosol-Assisted Self-Assembly of Mesostructured Spherical Nanoparticles,xe2x80x9d Lu, et al., Nature, 1999, vol. 398, 223; and xe2x80x9cMeasuring the Strength of Slurry Phase Heterogeneous Catalysts,xe2x80x9d Pham, et al., Powder Technol., 1999, vol. 103, 95.
The present invention pertains to, for example, a slurry phase Fischer-Tropsch (F-T) synthesis, a process used to convert energy reserves of coal and natural gas into liquid transportation fuels. At present, F-T synthesis is being practiced in South Africa for converting coal-derived syngas (CO+H2) into hydrocarbon waxes which are subsequently processed to the desired product slate.
CO+2H2xe2x86x92xe2x80x94CH2xe2x80x94+H2O
The preferred reactor type for F-T synthesis is a slurry phase bubble column reactor (SBCR) for F-T synthesis, which provides improved heat and mass transfer and operational simplicity in terms of catalyst loading and discharge. The catalyst commonly used for F-T synthesis is iron or cobalt. A potential disadvantage in using a SBCR involves the attrition of catalysts. Attrition becomes a problem with the F-T process when heavy waxy products are produced, and separation becomes difficult between the wax and nano-sized catalyst particles. The filter system can become plugged by the nano-particles, slowing down the recovery of liquid reaction products. Development of attrition-resistant iron catalysts is an urgent need for catalytic processes that operate in a liquid phase, and where separation of the catalyst from viscous liquid products is required. Several approaches have been proposed for the preparation of attrition-resistant catalytic materials.
U.S. Pat. No. 4,677,084 (xe2x80x9c""084 patentxe2x80x9d) discloses a process for making attrition-resistant catalysts, catalyst precursors and catalyst supports. The slurry consists of a catalyst material in an aqueous silicic acid solution that is spray-dried to form porous microspheres, having diameters ranging from 30 microns to 300 microns (1 micron equals 1xc3x9710xe2x88x926 meters). According to the ""084 patent, the relative amounts of particles and silicic acid are chosen so that the weight of the silica formed ranges from 3% to 15% of the total weight of particles and silica. The ""084 patent further states that the spray-dried microspheres are calcined to sinter and generate the catalyst. A calcination temperature is used that is below a temperature that is deleterious to the catalyst. The ""084 patent discloses attrition resistance measurements of the catalyst material, measured using a device where the catalyst is exposed to a high velocity gas jet.
A reference authored by H. Bergna that appears in Proc. Of the American Chemical Society symposium on Characterization and Catalyst Development, 1989, p. 55, (xe2x80x9cBergna referencexe2x80x9d) discloses embedding of catalyst particles in a continuous framework or skeleton of a hard and relatively inert material to enhance attrition resistance. The approach of the Bergna reference requires that the fraction of the hard phase volume approach 50% to form an attrition-resistant continuous framework within the grain pores. The Bergna reference also discloses use of sub-colloidal or very small colloidal particles capable of coalescing or sintering to form a hard eggshell thereby conferring a degree of attrition resistance with smaller amounts (10%) of a hard phase (silica). In this particular case, the silica must be distributed on the periphery of the particles, which is achieved by ensuring the following two conditions: (i) that the silica does not agglomerate during spray drying and (ii) that the silica particles migrate easily to the surface.
U.S. Pat. No. 5,221,648 discloses a process for making highly mesoporous catalytic cracking catalysts, particularly catalysts useful in fluidized catalytic cracking operations. Attrition resistance of these catalysts is measured in terms of the Davison Index, which is well known to practitioners in the field of catalysis.
U.S. Pat. No. 5,352,645 (xe2x80x9c""645 patentxe2x80x9d) discloses a process for making relatively strong, non-agglomerated porous uniform silica microspheres ranging in size from 1.0 microns to 50 microns. According to the ""645 patent, the microspheres are useful as catalytic supports in fluid bed and slurry applications, particularly in the catalytic process for the direct synthesis of hydrogen peroxide. The ""645 patent also discloses use of an aqueous sol of colloidal silica that is mixed with a soluble additive selected from ammonium citrate or urea, and subsequently spray-dried to form silica microspheres. The spray-dried powder is calcined to remove any organic residues and sintering of the primary particles is performed to develop strength and porosity. The ""645 patent discloses an attrition test that is performed by vibrating a mixture of the material in question and a granular alumina grinding medium in a heavy duty orbital sander. The particle size distribution of the material, separated from the grinding medium, is then analyzed using a Microtrac Model 158705 Analyzer, a typical particle size analyzer known in the art of particle size analysis.
U.S. Pat. No. 4,572,439 discloses a process for providing a rapid treatment for materials to be used in catalytic processes. Acoustical energy is applied to a slurry consisting of the material in question. After separating the aftrition-resistant particles from the liquid medium, the frangible particles are disintegrated to a fine powder and are separated from the liquid by filtration. The fines are agglomerated to suitably sized particles, and recycled to be slurried with the untreated material to be subjected to the attrition treatment.
U.S. Pat. No. 5,442,012 discloses a process for making encapsulated micro-agglomerated core/shell additives for PVC blends. A first core/shell impact modifier and a second core/shell processing aid are prepared separately by emulsion polymerization. The polymer particles are then admixed in emulsion form. The core/intermediate shell polymer particles are agglomerated, followed by sequential emulsion polymerization to form an encapsulating shell of hard polymer. This final shell can encapsulate more than one particle.
U.S. Pat. No. 5,633,217 discloses a process for making a high strength catalyst, catalyst support or adsorber, which makes use of silicone resin as an inorganic binder precursor dissolved in a cost-effective, non-flammable solvent.
The methods and materials disclosed and/or proposed in the aforementioned references for preparing attrition resistant catalysts fall short for preparing catalysts that are subject to phase transformations during use. For example, as disclosed in the Shroff et al., J. Catal., 156, p 185, 1995, reference, an iron catalyst start out in the form of an oxide and then gets converted to an iron carbide after activation in a Fischer-Tropsch reaction environment. The increase in density between the oxide and carbide phase leads to a break-up of the iron oxide to form nanoparticles of iron carbide. Hence, even if the catalyst could be prepared in attrition-resistant form, the phase changes tend to stress and weaken the catalyst during use.
The synthesis of mesoporous materials has attracted great interest in the field of catalysis, biomaterials, membrane and separation technology, and molecular engineering. Most mesoporous materials, such as silica, disclosed in the R. K. Iler, Chemistry of Silica, Wiley, N.Y., 1979, reference are amorphous, with pores that are irregularly spaced and broadly distributed in size. Recently, the Kresge et al., Nature, 359, p. 710, 1992, reference disclosed the synthesis of mesoporous silica by means of a liquid-crystal template mechanism, in which the silicate material forms inorganic walls between ordered surfactant micelles. Ordered arrays of cylindrical micelles are formed, with the silicate species occupying the spaces between the micelles. Once an ordered array of uniform channels is established, the organic material is burnt off to produce a stable crystalline mesoporous solid.
The synthesis proposed in the Kresge et al., Nature, 359, p. 710, 1992, reference involves placing a mixture of reagents in an autoclave for 48 hours. From an application standpoint, the synthesis approach is not suitable for large-scale production. An alternative approach was disclosed in the Lu et al., Nature, 398, p. 223, 1999 reference where they used an aerosol process for the synthesis of mesostructured spherical nanoparticles. The methodology of the Lu reference involves evaporation-induced surfactant self-assembly to synthesize silica thin films, membranes, particles, and nano-composite materials with highly ordered mesophase structures via dip coating or aerosol processes. A similar evaporation approach was also reported in the Bruinsma et al., Chem. Mater., 9, 2507, 1997 reference to yield mesoporous silica. In this latter work, the authors spray-dried powders, using a precursor solution consisting of cetyltrimethylammonium chloride (xe2x80x9cCTACxe2x80x9d), hydrochloric acid, tetraethoxysilane (xe2x80x9cTEOSxe2x80x9d), and water. These spray-dried mesoporous powders had structures ranging from hollow spheres to collapsed particles that were dependent on the precursor solution composition and drying conditions.
The approach disclosed in the Bruinsma et al. reference is disclosed further in U.S. Pat. No. 5,922,299 (xe2x80x9c""299 patentxe2x80x9d), which discloses a process for making mesoporous-silica films, fibers, and powders by evaporation of a solvent from the silica precursor solution. According to the ""299 patent, evaporation or rapid drying is accomplished by layer thinning, such as spin casting, liquid drawing and liquid spraying. When powders are produced by liquid spraying, micro-bubbles within the powder are hollow spheres with walls composed of mesoporous silica. The ""299 patent states that the mesoporous silica fibers may be impregnated with catalytically active metals for applications in structured catalytic packing. The metal addition to the mesoporous materials is via soluble metal salts such as halide or nitrate solutions only. The ""299 patent does not disclose the use of metal oxides or the addition of insoluble forms of metal catalysts. In F-T type reactions, the dispersion of a salt of iron throughout a catalyst/catalyst support often leads to adverse metal-support interaction that decreases the activity of the iron catalyst. The method of the ""299 patent also has an inherent limitation as to the amount of an iron phase that can be loaded into a mesoporous silica.
The use of mesoporous supports for Fischer-Tropsch synthesis has been recently reported in Patent Cooperation Treaty, International Publication No. WO 00/10698 (xe2x80x9c""698 PCT publicationxe2x80x9d). In this publication, a F-T process is disclosed. According to the ""689 PCT publication, the catalyst includes at least one catalytic metal for F-T reactions and a non-layered mesoporous support which, after calcination, exhibits an XRD pattern that has at least one peak at a d-spacing of greater than 18 xc3x85. The ""689 PCT publication discloses a process for metal loading in a range from 10% by weight to 30% by weight of metal. Supports having such low weight percents of metal are only suitable and industrially practical for metal catalysts having high activity, for example, cobalt and/or ruthenium catalysts. Indeed, the ""698 PCT publication does not teach a process for incorporating higher weight percentages of metal catalysts. For example, iron catalysts have a considerably lower activity when compared to cobalt and/or ruthenium catalysts; therefore, the weight percentage of an iron catalyst must be substantially greater than 30%. Such high weight percentage loading cannot be achieved with the approach disclosed in the ""689 PCT publication.
The present invention comprises a method and a composition of matter. In a preferred embodiment, the present invention comprises a method for encapsulating a dispersed insoluble compound in a mesoporous structure comprising the steps of: combining a soluble oxide precursor, a solvent and a surfactant to form a mixture; dispersing an insoluble compound in the mixture; spray-drying the mixture to produce dry powder; and calcining the powder to yield a porous structure comprising the dispersed insoluble compound. In this preferred embodiment, the method optionally comprises the step of introducing a precipitation control agent to the mixture to control precipitation of the oxide precursor. In general, such agents comprise acids, such as, but not limited to, HCl and HNO3. The method of this preferred embodiment also optionally comprises the step of cooling the mixture to control precipitation of the oxide precursor. According to a preferred embodiment, cooling comprises cooling to a temperature of approximately 10xc2x0 C. or less. To facilitate cooling to temperatures below the freezing point of the mixture, in a preferred embodiment, the solvent comprises at least one solvent comprising at least one antifreeze. For example, the present invention optionally comprises solvents comprising alcohols, such as, but not limited to, polyethylene glycol, ethylene glycol, ethanol, propylene glycol, and propanol, and other solvents that optionally comprise, for example, surfactants, such as nonionic surfactants. According to a preferred embodiment, solvent, or solvent mixtures, comprise a flash point below that of temperatures encountered in the spray drying step, alternatively, spray drying comprises spray drying in a substantially inert gas, or a gas that diminishes solvent ignition. According to the present invention, dispersing optionally comprises two steps comprising, for example, introducing and dispersing and/or combining and dispersing. For example, an insoluble compound is optionally combined with the components of the combining step and/or introduced to the mixture and then later dispersed. However, according to a preferred embodiment, dispersing comprises introducing at least one insoluble compound to the mixture. Of course, in some embodiments, the insoluble compound is not fully encapsulated, for example, but not limited to, some of the insoluble compound is optionally positioned at the outermost surface of a porous structure partide. Such embodiments are within the scope of the present invention; however, according to preferred embodiments, at least some of the insoluble compound is fully encapsulated in the porous structure.
In a preferred embodiment, the oxide precursor comprises an alkoxide of silicon and preferably a tetraethyl orthosilicate (TEOS) alkoxide of silicon. The method of the present invention optionally comprises at least one oxide precursor wherein the oxide precursor comprises at least one precursor selected from the group consisting of silicic acid, zirconium alkoxide, titanium alkoxide, and aluminum alkoxide.
According to a preferred embodiment of the present invention, the surfactant comprises at least one surfactant comprising at least one ammonium ion and preferably the surfactant comprises at least one surfactant, for example, but not limited to, cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC). According to the present invention, surfactant comprises at least one surfactant, including, but not limited to, amphoteric surfactants, nonionic surfactants, anionic surfactants, cationic surfactants and/or molecules that migrate to a surface between two phases and/or form assemblies.
In a preferred embodiment, the method of the present invention comprises at least one additional step of adjusting pH of the mixture, preferably after the combining step and preferably to a pH of less than approximately pH 5. This particular embodiment optionally comprises at least one adjusting step, preferably for adjusting the pH of the mixture to a pH less than approximately pH 5.
In a preferred embodiment, the insoluble compound comprises at least one oxide. In this embodiment, the at least one oxide optionally comprises at least one oxide, such as, but not limited to, iron oxide, titanium oxide, cobalt oxide and vanadium oxide. The insoluble compound also optionally comprises at least one zeolite, for example, but not limited to, catalytic zeolites. In a preferred embodiment, the insoluble compound comprises ZSM-5 zeolite.
In another preferred embodiment the insoluble compound optionally comprises at least one non-oxide phase wherein the at least one non-oxide phase optionally comprises at least one non-oxide phase selected from the group consisting of nitride and carbide, for example, but not limited to, at least one non-oxide phase optionally comprising molybdenum nitride, iron carbide and/or molybdenum carbide.
In a preferred embodiment, the dispersing step comprises sonication, for example, but not limited to, ultrasonication at frequencies greater than or equal to approximately 20 KHz. Of course lower frequencies are within the scope of the present invention. Power input is also a parameter that is adjustable to achieve a desired dispersion of the insoluble compound. Of course, as disclosed herein, use of more than one insoluble compound is within the scope of the present invention as is a plurality of dispersion steps and/or sonication steps. According to the present invention, an insoluble compound of the dispersing step preferably comprises submicron dimensions upon addition and/or upon dispersion in the mixture through use of dispersion mechanisms known in the art of particle science, such as, but not limited to, radiation induced dispersion and/or disruption, including sonic radiation and/or electromagnetic radiation.
In a preferred embodiment the mixture of the combining step forms a template for templating a mesoporous structure. In such an embodiment, a precipitation control agent of the introducing step allows for formation of a template for templating a mesoporous structure by delaying precipitation of the oxide precursor. Optionally, cooling, alone or in addition to a precipitation control agent, allows for formation of a template.
According to a preferred embodiment, the calcining step substantially removes the surfactant. Optionally, the calcining step removes other material not removed, or sufficiently removed, during the spray-drying step. For example, residual solvent, antifreeze, other agents and/or material is optionally removed during the calcining step.
The present invention also comprises a composition of matter comprising the porous structure comprising the dispersed insoluble compound of the inventive method. In a preferred embodiment, the composition of matter comprises a porous structure comprising a dispersed insoluble compound wherein the porous structure comprises pores formed by an oxide precursor templated on a surfactant template. In a preferred embodiment, the pores allow gas to access said dispersed insoluble compound. The insoluble compound optionally comprises a catalyst, which optionally changes phase during use as a catalyst and/or upon exposure to a reducing agent in the case of, for example, an insoluble compound comprising an oxide, preferably at least one metal oxide, such as, but not limited to, iron oxide. Reducing agents include, but are not limited to, hydrogen gas.
In a preferred embodiment, the composition of matter comprises a porous structure comprising an ordered porosity, preferably wherein the ordered porosity corresponds to an order from a surfactant template.
In a preferred embodiment, the composition of matter comprises resistance to attrition. In another preferred embodiment, the composition of matter comprises a Fischer-Tropsch catalyst.
In a preferred embodiment, the composition of matter comprises a porous structure comprising a phase-changed dispersed insoluble compound comprising nanoparticles (generally particles comprising submicron dimensions) wherein the porous structure comprises pores formed by an oxide precursor templated on a surfactant template. In such a preferred embodiment, the pores optionally comprise an average pore size that retains phase-changed nanoparticles (generally particles comprising submicron dimensions) of the dispersed insoluble compound within the porous structure.
The present invention comprises a method and a composition of matter.
A primary object of the present invention is to produce attrition resistant particles comprising at least one catalyst.
A primary advantage of the present invention is attrition resistance.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.